1. Field of the Invention
The present invention relates to an apparatus and a method for recognizing characters, and more particularly, to an apparatus and a method for recognizing characters by using the generalized Hough transform. This invention is especially favorably applicable to the recognition of identification information marked on semiconductor wafers, electronic circuit modules, and semiconductor or electronic devices such as circuit cards/boards.
2. Prior Art
Semiconductor wafers, electronic circuit modules, and semiconductor or electronic devices such as circuit cards/boards are usually provided with identification information such as product numbers, manufacturing numbers, and lot numbers by marking or printing. Such identification information is read, recognized, and used for various purposes, such as automatic control of manufacturing procedures, sorting of products, and quality control. Industrial character recognition is widely used for the identification not only of semiconductor devices and electronic devices but also of various products.
It is common in conventional industrial character recognition to separate a character to be recognized from an image and then to compare the separated character pattern with standard character patterns of templates. To recognize a wafer identification number of several digits, for example, the conventional character recognition entails separating those digits one by one and then performing pattern matching to each of the digits. In this recognition technique, the character separation phase and the recognition phase are presupposed to be separate processes. In this case, for a higher rate of recognition, it is essential that characters are properly separated and that the separated character patterns retain significant information required for character recognition. In the conventional recognition technique, in cases where adjacent characters contact; in cases where a character or its background involves scratches or noise; or in cases where a character is blurred or indistinct, not only is the pretreatment very troublesome, but also, it is very difficult to attain a higher recognition rate. Particularly where a workpiece, such as a semiconductor wafer, has undergone severe treatment involving etching, the identification information on the surface may suffer damage, or traces of the treatment may remain as noise indistinguishable from the identification information, with the result that inaccurate character recognition occurs. The highest rate of recognition of wafer identification numbers which can be expected from the conventional method of character recognition using pattern matching is no more than 95%.
Another problem attendant on character recognition is that in the conventional recognition technique, providing a separate recognition program from character type to character type is necessary. For example, the size, font, and quality of characters to be used for wafers may be different from those used for circuit modules, in which case it is usual to provide quite different programs. It would be very advantageous if a common recognition program could be applied to different character types with minimum modification.
"Generalizing the Hough Transform to Detect Arbitrary Shapes," by D. H. Ballard, Pattern Recognition, Vol. 13, No. 2 (1981), pp.111-122, describes a generalized Hough transform algorithm capable of extracting graphics of any shape. The Hough transform is a straight line detection algorithm, which was first proposed by P. V. C. Hough (U. S. Pat. No. 3,069,554) and later improved by R. O. Duda and P. E. Hart (R. O. Duda and P. E. Hart, "Use of the Hough Transform to Detect Lines and Curves in Pictures," Communications of the ACM, Vol. 15, No. 1, pp.11-15, 1972). The generalized Hough transform is an algorithm that develops the straight line detection Hough transform for the sake of detecting graphics of arbitrary shapes. In Ballard's paper, however, how to apply the generalized Hough transform to character recognition and how to perform efficient character recognition by use of the generalized Hough transform are not disclosed. Because the Hough transform entails a large amount of computations in transforming image information into a Hough space, that is, a parameter space, some device for heightening processing efficiency must be invented to utilize the generalized Hough transform in character recognition.